Clinton posted a tweet saying he received a copy of "41: A Portrait of My Father," Bush's biography of his dad, former President George H.W. Bush. At the end of the message, he included a hashtag asking the younger Bush why he has not yet joined Twitter:

There has recently been mounting speculation the Bush dynasty could eventually include a third president with growing speculation another one of George H.W. Bush's sons, former Florida Gov. Jeb Bush (R), could launch a White House bid in 2016.

Update (7:45 p.m.):This post was updated after President George H.W. Bush's spokesman informed us that he is on Twitter, but his son is not.